An example of a conventional marine power distribution and propulsion system is shown in FIG. 1. A series of diesel engines D are used to drive individual ac generators G. These supply ac power to a first medium voltage switchboard or busbar MVAC1 and a second medium voltage switchboard or busbar MVAC2. The medium voltage busbars are equipped with protective switchgear that comprise circuit breakers and associated controls and are represented in FIG. 1 by the x symbol. Power converters PC are used to interface the medium voltage busbars to electric propulsion motors PM that drive propellers. The medium voltage busbars are interconnected by protective switchgear.
Most of the ships services require low voltage and it is convenient to derive this from the medium voltage busbars by using a transformer. In the conventional marine power distribution and propulsion system shown in FIG. 1, a first low voltage switchboard or busbar LVAC1 is connected to the first medium voltage busbar MVAC1 through a first transformer T1 and protective switchgear. A second low voltage switchboard or busbar LVAC2 is connected to the second medium voltage busbar MVAC2 through a second transformer T2 and protective switchgear. The low voltage busbars are interconnected by protective switchgear. A series of unspecified electrical loads (labelled LVAC loads) can be connected to the low voltage busbars.
Although it is convenient to connect the low voltage busbars to the medium voltage busbars, such an arrangement normally leads to problematic coupling of harmonic distortion. In other words, harmonic distortion in the medium voltage busbars caused by the operation of the propulsion motors, for example, will be transferred through the transformers T1 and T2 to the low voltage busbars. If the electrical loads connected to the low voltage busbars require low harmonic distortion (i.e. a high quality of power supply (QPS)) then this may cause potential problems with their normal operation. Also, some of the loads connected to the low voltage busbars may themselves cause harmonic distortion in the low voltage busbars. This will be transferred through the transformers T1 and T2 to the medium voltage busbars.
In order to suppress such harmonic distortion, large and costly filters F are normally connected to the medium voltage busbars.
Another option for suppressing the harmonic distortion is to use phase-shifting transformers in place of the transformers T1 and T2.
In other marine power distribution and propulsion systems then a low voltage ac supply is employed and in this case the arrangement shown in FIG. 1 is still applicable but the first and second medium voltage busbars MVAC1 and MVAC2 simply operate at low voltage. This might be the case where the power rating of the propulsion motors PM is insufficient to warrant the use of a medium voltage ac supply. It is common for the propulsion motors PM to be supplied with a different value of low voltage than the low voltage supply that is provided to the LVAC loads. For example, propulsion loads may have a 690 V supply whereas ships service loads may have a 440 V supply.
In such low voltage systems it is common for larger auxiliary electrical loads such as thrusters, pumps and cranes, for example, to be fed by the same supply voltage and the propulsion motors. Although only propulsion loads are shown as being connected to the first and second medium voltage busbars MVAC1 and MVAC2, it will be readily appreciated that other electrical loads may also be connected to them.
Moreover, in some cases the propulsion converters PC that are used to interface the propulsion motors PM to the first and second medium voltage busbars MVAC1 and MVAC2, together with any other larger auxiliary electrical loads, may, at a cost and size penalty, incorporate additional technical features that minimise harmonic distortion in those busbars. The use of such additional technical features would eliminate the requirement for the filters F. Whichever approach is used to minimise harmonic distortion there is always a significant size and cost penalty to ensure that the QPS is maintained on the first and second medium voltage busbars MVAC1 and MVAC2 such that a satisfactory QPS is similarly maintained on the low voltage busbars LVAC1 and LVAC2.